Gas-Phase Reactivity of Naphthalene-Based Singlet Oxenium Cations

Six naphthyloxenium cations with singlet electronic ground states were generated in the gas phase in a linear quadrupole ion trap mass spectrometer and allowed to undergo reactions with seven reagents. All cations were unreactive toward water, cyclohexane, acetyl chloride, and benzylamine. Three cations (2-amino-1-oxo-, 4-amino-1-oxo-, and 5-amino-1-oxo-) were also unreactive toward ethylbenzene, dimethyl disulfide, and allyl iodide. The lack of reactivity of these cations is attributed to the extensive delocalization of their charge. However, the remaining cations, 1-oxo-, 2-oxo-, and 3-amino-1-oxonaphthyloxenium cations, underwent reactions with ethylbenzene, dimethyl disulfide, and allyl iodide. They produced a stable adduct upon interactions with ethylbenzene and dimethyl disulfide. Based on quantum chemical calculations, the most favorable position for nucleophilic attack by all studied nucleophiles in the unsubstituted naphthyloxenium cations is the carbon atom adjacent to the carbonyl group, while that for the amino derivative is either the same carbon atom or the carbon atom located in the position para to the carbonyl group. These preferred addition positions were supported by the calculated enthalpies for the addition of the free hydride anion. The cations react with allyl iodide via three pathways: allyl radical abstraction, allene abstraction, and formation of a stable adduct, whose mechanisms are discussed. The naphthyloxenium cations are substantially less reactive than the previously studied singlet quinolyloxenium cations.